Just about everything in a Windows NT\2000\XP\Vista environment is assigned a unique “security identifier” (SID). Every user on a machine and every computer in a domain has a long string of numbers and letters that make a each account or machine unique. This is great for security – for example, it prevents someone from installing Windows Server 2003 on a laptop and creating a “ghost domain” with the same users as your work domain. Because the SIDs are different between your work’s real domain and the hacker’s ghost domain, the fake accounts cannot get into the CEO’s email account or sensitive network shares.
However, this has long been a problem for IT folks when it comes to changing network configurations. If a company’s workstations need to be moved from a workgroup to a domain, or from an old domain to a new one, or even if the old domain controller crashed and had to be replaced… all of these instances cause the computer’s SID to change. And any “new” accounts will get a new SID. So this means that when you convert John Doe’s computer from a workgroup to a domain, he will have a new domain account and SID… which means that he will get a new profile on his computer… which means that all of his documents, bookmarks, settings, Start Menu entries, email, wallpaper(s), browsing history, saved passwords, and even his desktop will be different.

